The Response And Feedback Of Root-invading Fungi Of Major Plant Species To Sheep Grazing In Steppe,Eastern Gansu,China

Root-invading fungi are major components of grassland microbial community.Grazing is one of the most important ways to exploit natural grassland.In this study,we investigated how the intensity of sheep grazing affect root-invading fungi of dominant plant species Artemisia capillaris,Lespedeza davurica and Stipa bungeana in Steppe of Eastern Gansu China,and based on the framework of plant-soil feedbacks(PSFs),detected the potential role of root-invading fungi in grassland plant community.The main findings were as follows:1.The three dominant species harboured different root fungal communities,and the diversity of root-invading fungi displayed a hump-shaped trend with increasing grazing intensity,and was highest in plots of 2.7 sheep/hm~2 for all plant species.Fusarium tricinctum,F.oxysporum,Rhizoctonia solani and Setophoma terrestris were present in all plant species,and their mean isolation frequencies were more than5%.The isolation frequency of Fusarium in the three plant species showed species-specific trend as the grazing intensity increased.With A.capillaris and L.davurica,the occurrence of Fusarium species decreased with increasing grazing intensity,whereas with Stipa bungeana,its occurrence tended to increase as the grazing intensity increased.2.Several Fusarium species were all pathogenic to tested plant species,but their pathogenicities differed between the different Fusarium strains.In the greenhouse trial,compared to the control,F.tricinctum,F.oxysporum and F.redolens decreased biomass of A.capillaris,L.davurica and S.bungeana by 64%~79%,26%~45%and30%~42%,respectively.In the field trial,the strain of F.tricinctum obtained from L.davurica,and from S.bungeana,decreased seedling emergence of the host plant by72%and 76%,and decreased seedling emergence of the non-host plant by 74%and67%.By contrast,the strain of F.redolens obtained from L.davurica,and from S.bungeana,decreased seedling emergence of the host plant by 64%and 79%,but decreased seedling emergence of the non-host plant by 13%and 3%.3.Seed dormancy responded differently to the inoculation of F.tricinctum.In the plate trial,nondormant seeds of S.bungeana and L.davurica inoculated with F.tricinctum developed white tufts of mycelium on the radicles of germinating seeds causing them to quickly die,but dormant seeds of the two plant species remained intact.In contrast,in the field inoculation with F.tricinctum did not cause higher mortality of nondormant than dormant seeds but resulted in higher percentages of seedling death before they emerged from soil than the controls.These findings suggest that dormancy protects seeds from being attacked by some pathogens by preventing germination,but the protection was lost once germination has commenced.4.Grazing intensity generated abiotic and biotic PSFs for the plant population.Regardless of soil sterilization,biomass of all plant species and the concentration of soil inorganic nitrogen increased with increasing grazing intensity,suggesting that the intensity of grazing creates consistent abiotic PSFs for different plant populations.Biomass of A.capillaris and S.bungeana in sterilized soil was significantly higher than that in unsterilized soil from plots of 0 sheep/hm~2 and 8.7 sheep/hm~2 respectively,and biomass of L.davurica in unsterilized soil from all grazing intensities was significantly higher than that in sterilized soil,suggesting that each grazing intensity generates species-specific biotic PSFs for different plant populations.As for L.davurica,root colonization by AM fungi was high for all grazing intensities,standing at 40%,compared with 23%and 17%for A.capillaris and S.bungeana,respectively.The occurrence of F.tricinctum tended to decrease with increasing grazing intensity for A.capillaris and L.davurica,while it showed an opposite trend for S.bungeana.These findings suggest that root fungal communities were key drivers of biotic PSFs.5.Grazing intensity generated abiotic and biotic PSFs for the plant community.Independent of soil sterilization,the total community biomass increased significantly as the grazing intensity increased,suggesting that the intensity of grazing creates positive abiotic PSFs for the plant community.Within each grazing intensity,total community biomass were significantly greater in unsterilized than in sterilized soil,suggesting that each grazing intensity generates positive biotic PSFs for the plant community.Compared to the control,inoculation with Fusarium decreased the community biomass by 27%,but inoculation with AM fungi increased it by 19%.The presence of Fusarium significantly decreased the biomass proportion of dominant plant species A.capillaris and S.bungeana,but the presence of AM fungi increased the biomass proportion of subordinate species Dodartia orientalis and Oxytropis racemosa in the plant community.These findings suggest that pathogenic fungi potentially promote species coexistence by limiting the performance of dominant species,and AM fungi have the potential to promote plant species coexistence by facilitating the performance of subordinate species.